K. Pękała

Magnetocaloric and transport study of poly- and nanocrystalline composite manganites La0.7Ca0.3MnO3/La0.8Sr0.2MnO3

Magnetocaloric and transport properties are reported for novel poly- and nanocrystalline double composite manganites, La0.8Sr0.2MnO3/La0.7Ca0.3MnO3, prepared by the sol-gel method. Magnetic field dependence of magnetic entropy change is found to be stronger for the nano- than the polycrystalline composite. The remarkable broadening of the temperature interval, where the magnetocaloric effect occurs in poly- and nanocrystalline composites, causes the relative cooling power (RCP(S)) of the nanocrystalline composite to be reduced by only 10% compared to the Sr based polycrystalline phase. The RCP(S) of the polycrystalline composite becomes remarkably enhanced. The low temperature magnetoresistance is enhanced by 5% for the nanostructured composite.

The Determination of the Grain Boundary Width of Ultrafine Grained Copper and Nickel from Electrical Resistivity Measurements

The grain size distribution of copper and nickel processed by severe plastic deformation was studied by transmission electron microscopy. The temperature dependence of electrical resistivity in the temperature range from 20 to 300 K is presented. The tunnel model of electrical resistivity of grain boundaries is discussed. The width of the potential barrier at grain boundaries in copper and nickel is determined. Larger values of potential barriers are noted in ultrafine grained materials in comparison to the crystallographic width of grain boundaries.

Crystallization and relation between the temperature coefficient of resistivity and stability of NixZr1−x metallic glasses

AbstractElectrical resistivity and magnetic susceptibility of amorphous NixZr1−x alloys obtained by the “melt spinning” method have been measured in order to study crystallization kinetics. The correlation between the degree of negativity of the temperature coefficient of resistivity and the stability of amorphous alloys was observed. The studied relation between stability and the coefficient

Nanocrystallization of Al92Sm8 studied by electrical resistivity—formation of monatomic Sm spherical layer

\beta = \left( {\frac{{\varrho _a }}{{\varrho _c }} - 1} \right)_{T_c }

Electron transport study of nanocrystallization in FeSiB based alloys

indicated that β may be treated as a measure of the structural changes occuring during crystallization.

Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses

Comparative study of magnetotransport and magnetic properties in strong magnetic fields up to 47 T is reported for nanocrystalline and polycrystalline mixed valence manganites La0.8Sr0.2MnO3. The low temperature resistivity enhancement observed only in nanocrystalline manganite is ascribed to the electrostatic blockade of charge carriers between the fine grains/crystallites. Small polaron model is found to describe the high temperature conductivity both in poly- and nanocrystalline manganites. The quantitative analysis supplies the low temperature blocking energy barrier and polaron activation energy. An evolution of resistivity with temperature and magnetic field reveals the intrinsic and extrinsic contributions to large negative magnetoresistance effect. The magnetoresistance is found to be correlated with a squared magnetization.

Electron Structure, Stability and Nanocrystallization of Al - Based Amorphous Alloys

We present a model describing the temperature dependence of the resistivity of the Al92Sm8 system during formation of fcc-Al nanocrystals. We take into consideration changes in resistivity of the amorphous phase due to the changing content of Al, the varying density of the crystallization nuclei, the creation of the Sm layer on the grain surface and decomposition appearing during annealing of the amorphous phase. Our calculations exhibit a better agreement with experimental electrical resistivity data for the nanocrystallization model considering previous decomposition of amorphous structure and formation of a monatomic Sm layer on grain boundaries than for a model of nanocrystallization on quenched-in nuclei with growth controlled by diffusion with a diffusion field impingement.

Crystallization of Nanocrystalline Alloys Studied by Electron Transport, Magnetic and Dilatometric Methods

The MA process of Fe 50 Al 50 powder mixture resulted in the formation of supersaturated bcc Fe(Al) solid solution, characterized by a minimum crystallites size and maximum strain level of 5 nm and 0.65 %, respectively. The lattice parameter of the Fe(Al) reached 0.2935 nm. The magnetic measurements revealed ferromagnetic state of the alloy, with Curie temperature about 830 K. From the Mossbauer spectra the average hyperfine magnetic field of about 22 T, connected with the bcc Fe(Al), was obtained. Heating the MA samples in the DSC up to 1000 K resulted in the formation of the ordered FeAl intermetallic compound with mean crystallites size about 10 nm.